A large number of oxide catalysts for use in the production of unsaturated aldehyde as a main component have been reported so far. For example, the oldest oxide catalyst was found by Standard Oil Co. of Ohio and is known as a composite oxide catalyst comprising Mo and Bi as essential components. Patent Literature 1 describes a catalyst by focusing on Mo, Bi, Ce, K, Fe, Co, Mg, Cs, and Rb as metals constituting the catalyst.
The method for producing unsaturated aldehyde is used in, for example, a method for producing (meth)acrylate such as methyl acrylate or methyl methacrylate through oxidative esterification reaction using at least one starting material selected from the group consisting of propylene, isobutylene, isobutanol, and t-butyl alcohol and an intermediate unsaturated aldehyde such as acrolein or methacrolein. This method for producing (meth)acrylate is also known as a so-called direct methyl esterification process consisting of two reaction steps or as a so-called direct oxidation process consisting of three reaction steps. The direct oxidation process produces (meth)acrylate by three steps (see e.g., Non-Patent Literature 1). The first oxidation step of the direct oxidation process is a step of producing unsaturated aldehyde such as acrolein or methacrolein through the gas-phase catalytic oxidation reaction of at least one starting material selected from the group consisting of propylene, isobutylene, and t-butyl alcohol with molecular oxygen in the presence of a catalyst. The second oxidation step of this process is a step of producing (meth)acrylic acid through the gas-phase catalytic oxidation reaction of the unsaturated aldehyde obtained in the first oxidation step with molecular oxygen in the presence of a catalyst. The final esterification step is a step of further esterifying the (meth)acrylic acid obtained in the second oxidation step to obtain (meth)acrylate. The esterification using alcohol such as methanol can yield methyl acrylate or methyl methacrylate.
By contrast, the direct methyl esterification process consists of two catalyst reaction steps: a first reaction step of producing unsaturated aldehyde such as acrolein or methacrolein through the gas-phase catalytic oxidation reaction of at least one starting material selected from the group consisting of propylene, isobutylene, isobutanol, and t-butyl alcohol with molecular oxygen-containing gas; and a second reaction step of reacting the unsaturated aldehyde thus obtained with alcohol such as methanol and molecular oxygen to produce (meth)acrylate such as methyl acrylate or methyl methacrylate at once.
Reaction systems using such oxide catalysts include fixed-bed, fluidized-bed, and moving-bed reaction systems. Of them, the fixed-bed reaction system is frequently adopted industrially by virtue of the following advantage: high reaction yields can be achieved by feed gas flowing in a state close to extrusion flow.
The fixed-bed reaction system, however, has low heat conductivity and is therefore unsuitable for exothermic reaction or endothermic reaction that requires heat removal or heating. Particularly, severe exothermic reaction, such as oxidation reaction, where the temperature suddenly rises, disadvantageously gets beyond control of the reaction system, possibly resulting in runaway reaction. In addition, such a sudden rise in temperature damages the catalyst, resulting in the unfavorable early degradation of the catalyst.
By contrast, the fluidized-bed reaction system has high heat conductivity because catalyst particles vigorously flow in the reactor. Thus, the temperature in the reactor is kept almost constant even during reaction in which heat is largely generated or absorbed. The fluidized-bed reaction system can advantageously prevent the reaction from excessively progressing. This reaction system also has the advantage that, because of the reduced local accumulation of energy, feed gas in an explosive range can be reacted so that the starting material concentration is increased to improve productivity. Thus, the fluidized-bed reaction system is suitable for the catalytic oxidation reaction of olefin and/or alcohol, which is high exothermic reaction. In spite of these known advantages of the fluidized-bed reaction system, Patent Literatures 2 and 3, for example, state that use of fixed-bed catalysts is generally preferred for converting unsaturated hydrocarbon to unsaturated aldehyde. These literatures state that the catalysts described therein may be used in any of fixed-bed, moving-bed, and fluidized-bed methods for producing unsaturated aldehyde through the catalytic oxidation reaction of olefin and/or alcohol, but make no specific mention about reaction systems other than the fixed-bed one.
Although naphtha pyrolysis is mainstream as a method for producing diolefin such as 1,3-butadiene, there is a growing demand for production based on gas-phase oxidation reaction along with a recent shift to alternative resources to petroleum. Examples of the method for producing diolefin by use of the gas-phase oxidation reaction include methods which involve subjecting monoolefin having 4 or more carbon atoms, such as n-butene or isopentene, and molecular oxygen to catalytic oxidative dehydrogenation reaction in the presence of a catalyst to produce conjugated diolefin, such as 1,3-butadiene or isoprene, corresponding to the monoolefin. As for the catalyst used in such reaction, for example, Patent Literature 4 describes an oxide catalyst comprising Mo, Bi, Fe, Ce, Ni, Mg, and Rb as a catalyst for the oxidative dehydrogenation reaction of monoolefin.
A known method for producing unsaturated nitrile such as acrylonitrile or methacrylonitrile involves reacting one or more selected from the group consisting of propylene, isobutylene, isobutanol, and t-butyl alcohol, with molecular oxygen and ammonia in the presence of a catalyst. This method is widely known as an “ammoxidation process” and is currently practiced at an industrial scale.
Catalysts for use in the ammoxidation process have been diligently studied with the aim of further efficiently carrying out the method for producing unsaturated nitrile at an industrial scale. For example, Mo—Bi—Fe—Ni or Mo—Bi—Fe—Sb composite metal oxide catalysts are known as such catalysts for ammoxidation. Composition composed of these essential metals supplemented with other components has also been frequently studied with the aim of improving performance. For example, Patent Literature 5 discloses a catalyst comprising molybdenum, bismuth, iron, cerium, and nickel supplemented with other components. Also, Patent Literature 6 discloses a catalyst comprising molybdenum, bismuth, iron, antimony, nickel, and chromium supplemented with other components.
According to Non-Patent Literature 2, the disordered phase, which is a metastable structure, refers to a structure containing Mo sites randomly substituted by Fe, for example, in the case of a Bi—Mo—Fe 3-component composite oxide, and is characterized in that Mo and Fe atoms form the same oxygen tetrahedron structure. On the other hand, the ordered phase, which is a stable structure, has the same composition as that of the disordered phase, but structurally differs therefrom, and is obtained by heat treatment at a higher temperature than that for the disordered phase. In the ordered phase, Fe and Mo atoms individually form tetrahedrons. This means that the Fe atom forms an oxygen tetrahedron while the Mo atom forms another oxygen tetrahedron, aside from the Fe atom. Non-Patent Literature 2 states that a Bi3Fe1Mo2O12 disordered phase is formed at 450° C., but undergoes phase transition to the ordered phase at a reaction temperature of 475° C.
1) Disordered Phase Bi3Fe1Mo2O12 
FIG. 1 shows the crystal structure of the disordered phase Bi3Fe1Mo2O12 described in Non-Patent Literature 2. This disordered phase is a tetragonal system of scheelite-type crystal (CaWO4 type) with two equal side lengths and three axial angles of 90 degrees in the lattice constant of a unit cell (A=B≠C and α=β=γ=90 degrees). This phase has two sites: the X sites, which are enclosed in oxygen tetrahedrons, and the Y sites, which are not enclosed by oxygen atoms. The X sites are occupied by Mo and Fe either at random or with a certain probability distribution. The Y sites are occupied by Bi and other elements or lattice defects either at random or with a certain probability distribution. In each layer in the plane AB, the X sites and the Y sites form planar square lattices with lengths equal to those of the lattice constants in the A-axis and B-axis directions, respectively, and occupy positions displaced in the A-axis and B-axis directions, respectively, by ½ of the lattice constant in plane. These layers in the plane AB are stacked in the C-axis direction while repetitively displaced by (A/2, 0) and (0, B/2), respectively. In this stacking, the oxygen tetrahedrons around the X sites are placed while rotating about the C-axis by 90 degrees with their incorporated atoms centered.
FIG. 3 shows the X-ray diffraction (XRD) of the disordered phase Bi3Fe1Mo2O12. This disordered phase exhibits single peaks at least in the 18.30°±0.2° (101), 28.20°±0.2° (112), 33.65°±0.2° (200), and 46.15°±0.2° (204) planes in the range of X-ray diffraction angles 2θ=10° to 60° measured by crystal X-ray diffraction (XRD).
2) Ordered Phase Bi3Fe1Mo2O12 
For comparison, the crystal structure of the ordered phase Bi3Fe1Mo2O12 is shown in FIG. 2. This ordered phase is a monoclinic crystal system having a distorted scheelite structure with different side lengths in the lattice constant of a unit cell. Two out of three angles formed by basic vectors are 90 degrees and the other angle is different (A≠B≠C, α=γ=90 degrees, and β≠90 degrees). The ordered phase has three sites: two unequivalent sites X1 and X2, which are enclosed in oxygen tetrahedrons, and the Y sites, which are not enclosed by oxygen atoms. The X1 sites are occupied by Mo and other elements or lattice defects. The X2 sites are occupied by Fe and other elements or lattice defects. The Y sites are occupied by Si and other elements or lattice defects.
3) Structural Difference between Disordered Phase Bi3Fe1Mo2O12 and Ordered Phase Bi3Fe1Mo2O12 
In the disordered phase Bi3Fe1Mo2O12, the X sites or the Y sites are equivalent to one another, or elements of different types coordinate at random. In the ordered phase, the X sites or the Y sites are occupied regularly and distinctively by elements of different types or defects so that these two types of sites are differentiated. The ordered phase therefore exhibits peak splitting in the X-ray diffraction, whereas the disordered phase is characterized in that single peaks are detected (indicated by the arrows in FIG. 3). In measurement in the range of X-ray diffraction angles 2θ=10° to 60°, the peak in the 18.30°±0.05° (101) plane of the disordered phase Bi3Fe1Mo2O12 is split into 18.15°±0.05° (310) and 18.50°±0.05° (111) planes; the peak in the 28.20°±0.05° (112) plane of the disordered phase is split into 28.05°±0.05° (221) and 28.40°±0.05° (42-1) planes; the peak in the 33.65°±0.05° (200) plane of the disordered phase is split into 33.25°±0.05° (600) and 34.10°±0.05° (202) planes; and the peak in the 46.15°±0.05° (204) plane of the disordered phase is split into 45.85°±0.05° (640) and 46.50°±0.05° (242) planes.